Corrosion resistant metal topcoats (i.e., the outermost coating) have been applied to a variety of substrates for many years. A particularly effective such topcoat comprises electrolessly deposited nickel containing about 3% phosphorus (hereafter Ni-P alloy). Such Ni-P alloy topcoats have demonstrated corrosion resistance superior to that of pure nickel and are used to provide cosmetic protection for a variety of products including appliance and interior automobile components. Ni-P coatings, however, are expensive, owing to their high nickel content, and are ineffective topcoats for iron or steel destined for use in environments where galvanic corrosion is prevalent and disruption of the integrity/continuity of the topcoat is possible. In this latter regard, Ni-P coatings are galvanically more noble than the underlying iron/steel which results in accelerated localized electrochemical consumption of the underlying steel/iron substrate at sites where any breaks/discontinuities occur in the topcoat. Such localized consumption ultimately results in perforation of the substrate at the corrosion site.
Automobile exterior body parts (e.g., fenders, door panels and the like) are among the most difficult parts to protect from corrosion because of the environment in which they must survive and their susceptibility to surface damage tending to create corrosion sites. Steel used for such applications is commonly electrogalvanized (i.e., cathodized @ current densities of about 0.25 A/cm.sup.2 to about 1.0 A/cm.sup.2) in strip plating reactors wherein substantially continuous lengths of steel strip are advanced rapidly through the electrogalvanizing bath (i.e., electrolyte) in such a manner as to maintain high zrnc ion-concentrations at the surface of the strip and prevent the formation of a thick zinc-ion-depleted diffusion layer thereat during plating. Turbulent or high speed laminar flow are adequate for this purpose and will vary from one reactor to the next. Steel electrogalvanized with binary zinc-nickel alloys has received considerable attention from the steel industry for automobile body applications. Such binary zinc-nickel electrogalvanizing alloys generally contain about 10% to 20% Ni and are a mixture of the .eta. (eta) and .gamma. (gamma) phases of the alloy. While such binary zinc-nickel electrogalvanizing alloys are not as corrosion resistant as the Ni-P alloys, they are electrochemically less noble than the underlying iron/steel substrate and therefore sacrificially protect the substrate from perforation corrosion wherever the substrate might be exposed. Such nickel-containing zinc electrogalvanizing alloys, however, are not as corrosion resistant as Ni-P alloys and eventually undergo a phase transition during the corrosion/dissolution process which transforms their initially less noble, sacrificial character into one that is more noble than the underlying steel. This nobility reversal occurs when the zinc content falls in the range of about 65-35% by weight zinc at which time the now more noble coating contributes to accelerated localized corrosion at the site of any breaks or disruption therein.
It would be desirable to obtain a protective topcoat: which has a lower nickel content than Ni-P alloy; which has a corrosion-resistance approaching that of Ni-P alloy; and which is easy to apply to the surface of a substrate.
It is an object of the present invention to provide a unique, corrosion resistant Zn-rich, nickel electrodeposit for the improved corrosion protection of underlying substrates. It is another object of the present invention to provide a unique, multi-layer coating system for the corrosion protection of iron or steel including a topcoat of such unique electrodeposit atop a sacrificial zinc layer intermediate the substrate and the topcoat and a Ni-rich, zinc-based alloy buffer layer intermediate the aforesaid sacrificial layer topcoat. It is a still further object of the present invention to provide a current efficient method for forming a continuous layer of the aforesaid corrosion resistant Zn-rich, nickel electrodeposit. These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.